Conventionally, to grind a crystal piece for instance to such an extent that it obtains a desired natural frequency, an approximate grinding time required for the crystal piece to reach the thickness for a desired natural frequency is first measured in advance. Then, crystal pieces are mounted on a grinding apparatus to undergo grinding for the period of said measured time, and after that taken out to make measurement of the natural frequency of each crystal piece. If the frequency proves not to have reached the desired natural frequency, re-grinding is performed. Therefore, the frequency adjustment requires longer time, and much labor is required since each crystal piece has to be taken out every time measurement of frequency is made.